1. Field of the Invention
This invention relates generally to sliding-type bearings, and more particularly to those having a sintered powder metal bronze bearing material applied to a steel backing, such as used in engine bearings.
2. Related Art
It is common in sliding bearing applications, including engine bearings, to bond a powder metal bronze alloy to a steel backing to journal a crankshaft or the like. The copper tin alloy matrix provides a strong bearing surface that can withstand the loads subjected on the bearing in use. Such bearings must also exhibit suitable wear and seizure resistance properties, and for this purpose it is common to add a certain additional alloying constituents, including lead to the bronze matrix. Lead acts as a lubricant to the bearing surface. It is also common to add a thin coating of tin to the bearing surface to further enhance the wear and seizure characteristics of the bearing.
Due to environmental considerations, various substitutes for lead have been explored, but to date none have demonstrated the ability to truly substitute for lead without unduly sacrificing the strength, wear, seizure and various other properties in many sliding bearing applications, including engine bearings.
Applicants' have found that bismuth, when prealloyed with powder metal bronze in a controlled amount along with a controlled amount of phosphorus can be sintered and bonded to a steel backing to provide a resultant sintered bronze steel-backed engine bearing whose physical properties are equal to or better than that of bronze-lead bearings while also exhibiting wear and seizure resistant properties equal to or exceeding those of steel-backed powder metal bronze-lead engine bearings.
An engine bearing constructed according to the Applicant's own prior invention (namely U.S. Pat. No. 6,746,154) comprises an essentially lead-free bronze powder metal bearing material bonded to a steel backing. The bearing material consists essentially of 8 to 12% by weight of tin, 1 to less than 5% by weight of bismuth, and 0.03 to 0.8% by weight of phosphorus, with the balance being made up essentially of copper.
Bronze-bismuth-phosphorus engine bearings constructed according to the prior invention exhibit physical properties of tensile strength greater than or equal to 400 MPa, yield strength greater than or equal to 290 MPa, elongation greater than or equal to 10% and hardness greater than or equal to 130 Hv 0.5/15. By way of comparison, a traditional copper-tin-lead bearing having 10 wt. % tin, and 10 wt. % lead exhibits, on average, a considerably lower yield strength of 223 MPa, a comparable tensile strength of 301 MPa, a reduced elongation of about 8%, and a reduced hardness of about 96 HV 0.5/15. By way of further comparison, an identical engine wear test was conducted on bronze-bismuth-phosphorus bearings prepared according to the prior invention against more traditional copper-tin-lead bearings of the type described above. The more traditional copper-tin-lead engine bearings exhibited a loss of about 12 microns due to wear, whereas bearings prepared according to the invention exhibited an average of about 10-11 microns, demonstrating that the wear and seizure resistance of bearings according to the invention are at least as good, if not better than that of the traditional copper-tin-lead engine bearings.
It has been surprisingly found that bearings prepared according to the Applicant's prior invention exhibit the beneficial property, when subjected to frictional sliding loading in use, of having a certain amount of tin, which is in solid solution with the copper migrate to the bearing surface, with the result being that a tin-rich layer is formed at the bearing surface which was not present after sintering or prior to installation and use of the bearing. This migration of tin and formation of a highly tin-rich layer at the bearing surface greatly increases the lubricity of the bearing and thus contributes to enhanced wear and seizure resistant characteristics of the bearing once the bearing is put into use. Such tin migration has not been observed in traditional copper-tin-lead bearings, nor with other proposed lead substitutes, such as nickel. While not entirely understood, it is believed that, when subjected to frictional sliding loading, the bismuth reacts with the tin in the matrix and effectively mobilizes the tin, drawing it to the bearing surface. Following testing, a visual inspection of the engine bearings prepared according to the invention showed the bearing surface to have a lustrous, tin-colored bearing surface, and a chemical analysis conducted on the bearing showed a considerably higher concentration of tin at the surface than in portion of the copper-tin matrix below the surface, which remained uniform in its tin concentration.
This surprising property of tin migration has the benefit of eliminating or minimizing the need to apply a tin flash coating to the bearing surface prior to putting the bearing into service. The elimination of the flash coating step saves time and equipment and simplifies as well as lowers the cost of making engine bearings.
The elimination of lead from the engine bearings has the advantage of providing a more environmentally compliant engine bearing, and the substitution thereof with bismuth in the manner called for by the Applicant's prior invention has the advantage of providing the same or better strength and wear/seizure resistant properties without requiring substantial changes in the way engine bearings are made. As such, engine bearings prepared according to the prior invention are readily adaptable to new applications or existing applications that would otherwise call for copper-tin-lead bearings, and a manufacturer of bearings according to the invention can adapt to the making of such bearings without requiring new or substantially modified manufacturing equipment, and perhaps eliminate some of the steps and equipment normally associated with the manufacture of traditional copper-tin-lead bearings.
According to a further aspect of the Applicant's prior invention (U.S. Pat. No. 6,746,154), particular benefits have been realized when a copper-tin-bismuth sintered compact is produced from a blend of water-atomized copper-bismuth powder and gas-atomized copper-tin powder. Again, while not entirely understood, it is believed that the process by which the powders are made contributes to the mobilization of the tin onto the bearing surface.
Other related art of notable mention may include U.S. Pat. No. 6,905,779 assigned to Daido Metal Company, Ltd. This patent is directed toward improved seizure resistance while unconcerned in any way with alloys containing bismuth or issues associated with wear resistance during the break-in period. Here, a mechanical alloying technique is used to achieve a uniform hard particle distribution in the material composition.
Still other related art includes GB2355016A assigned to Daido Metal Company Ltd. which teaches a copper alloy that comprises 0.5-15 mass % tin, 1-20 mass % bismuth and 0.1-10 volume % hard particles having an average size of 1-45 μm. The bismuth exists as a bismuth phase dispersed through the alloy. The hard particles may comprise one or more of borides, silicides, oxides, nitrides, carbides and/or an intermetallic compound. The alloy may further comprise not more than 40 mass % of Fe, Al, Zn, Mn, Co, Ni, Si and/or P. It may also further comprise not more than 20 volume % of one or more of MoS2, WS2, BN and graphite. The bearing alloy material is made by sintering an admixture of pure copper, tin and bismuth powders and various hard particle powders. This patent teaches that the hard particles mentioned co-exist with the bismuth phase in the copper matrix. The size of the bismuth phase is generally larger than the diameter of the hard particles, as the particles are located in the bismuth phase.
Still other related art includes US 2006/0000527 assigned to Taiho Kogyo Co. Ltd. which teaches a Cu—Bi based alloy, which can simultaneously attain a high level of the compatibility, fatigue resistance and corrosion resistance required for the bearing of a fuel injection pump. The '527 patent teaches a lead-free bearing, which contains from 1 to 30 mass % of Bi and from 0.1 to 10 mass % of hard particles having from 10 to 50 μm average particle diameter, the balance being Cu and unavoidable impurities, and further the Bi phase dispersed in the Cu matrix has an average particle diameter smaller than that of the hard particles. The Cu—Bi alloy is pulverized by an atomizing method. The resultant Cu—Bi powder is mixed with hard-particle powder and powder of the other metallic components. The components other than the hard-particle powder may be in the form of an alloy powder prepared by the atomizing method. The hard particles may be carbides such as Cr2C3, Mo2C, WC, VC and NbC, and preferably Fe2P, Fe3P, FeB, Fe2B, and Fe3B.
Notwithstanding the advantages associated with the use of copper-tin-bismuth bearing materials as described in U.S. Pat. No. 6,746,154, occasional early bushing wear may be experienced when using sintered bushings made from the composition. Such wear problems typically manifest during the initial break-in period of service. Despite the uses of certain combinations of hard particles suggested above, further bearing material improvements are desirable, particularly to improve wear and seizure resistance of the bearing, including early wear and seizure resistance, while also maintaining a suitable combination of mechanical and physical properties, such as tensile strength and ductility or elongation. It is also desirable to affect the necessary improvements while minimizing the amount of additional hard particle powder materials used, since these materials frequently have an associated cost that is greater than the cost of the alloy powders they replace in the sintered bearing material.